This invention relates to decontamination apparatus and more particularly to apparatus for decontaminating heat exchange tubes in a nuclear steam generator.
A typical nuclear steam generator comprises a vertically oriented shell, a plurality of U-shaped tubes disposed in the shell so as to form a tube bundle, a tubesheet for supporting the tubes at the ends opposite the U-like curvature, and a dividing plate that cooperates with the tubesheet forming a primary fluid inlet plenum at one end of the tube bundle and a primary fluid outlet plenum at the other end of the tube bundle. The primary fluid having been heated by circulation through the nuclear reactor core enters the steam generator through the primary fluid inlet plenum. From the primary fluid inlet plenum, the primary fluid flows upwardly through first openings in the U-tubes near the tubesheet which supports the tubes, through the U-tube curvature, downwardly through second openings in the U-tubes near the tubesheet, and into the primary fluid outlet plenum. At the same time, a secondary fluid known as feedwater, is circulated around the U-tubes in heat transfer relationship therewith, thereby transferring heat from the primary fluid in the tubes to the secondary fluid surrounding the tubes causing a portion of the secondary fluid to be converted to steam. Since the primary fluid contains radioactive particles and is isolated from the secondary fluid by the U-tube walls and the tubesheet, it is important that the U-tubes and tubesheet be maintained defectfree so that no breaks will occur in the U-tubes or in the welds between the U-tubes and the tubesheet, thus preventing contamination of the secondary fluid by the primary fluid.
Occasionally, it is necessary to inspect or repair the U-tubes or the tubesheet welds by way of access through the primary fluid inlet and outlet plena. For this purpose, manways are provided in the vertical shell so that working personnel may enter the inlet and outlet plena to perform operations on the U-tubes and tubesheet. However, since the primary fluid which is generally water contains radioactive particles, the inlet and outlet plena become radioactive, which thereby limits the time that working personnel may be present therein. In addition, since the primary fluid is conducted through the U-tubes, the U-tubes also become contaminated. Accordingly, it would be advantageous to be able to perform operations on the U-tubes and tubesheet without being exposed to high levels of radiation. Consequently, it is important to decontaminate the inlet and outlet plena together with a portion of the U-tubes so that working personnel may enter the inlet and outlet plena of the nuclear steam generator to perform operations thereon.
Therefore, what is needed is apparatus that is capable of decontaminating the U-tubes so as to reduce the radiation level in the inlet and outlet plena of the nuclear steam generator so that work may be performed thereon.